1. Field of the Invention
The present invention relates to an apparatus for measuring an optical property of a sample, and specifically relates to an apparatus for measuring an optical property of a sample containing fluorescent material.
2. Description of the Related Art
Generally, when a measurement sample containing fluorescent material (hereinafter referred to as "fluorescent sample") is irradiated by white light I, the visual characteristics are expressed by the total spectral radiant factor .beta..sub.t,I (.lambda.). Total spectral radiant factor .beta..sub.t,I (.lambda.) is the ratio of spectral radiance illuminated and observed under the same conditions at wavelength .lambda. of a complete diffuse reflection surface and an observed fluorescent sample, or otherwise the ratio of the flux intensity at wavelength .lambda. reflected from the complete diffuse reflection surface and fluorescent sample in the same solid angle of the same direction when illuminated under the same conditions, and is expressed by Equation 1. EQU .beta..sub.t,I (.lambda.)=S(.lambda.)/S.sub.0 (.lambda.) (1)
In this equation, S(.lambda.) is the intensity at wavelength .lambda. of the radiant light from the fluorescent sample, and S.sub.0 (.lambda.) is the intensity at wavelength .lambda. of the radiant light reflected from the complete diffuse reflection surface.
The total spectral radiant factor .beta..sub.t,I (.lambda.) is expressed by Equation 2 when the reflecting spectral radiance factor of the reflected light component of a fluorescent sample is expressed as .beta..sub.o,I (.lambda.), and the fluorescent spectral radiance factor of the fluorescent light component reflected from a fluorescent sample is expressed as .beta..sub.f,I (.lambda.). EQU .beta..sub.t,I (.lambda.)=.beta..sub.0,I (.lambda.)+.beta..sub.f,I (.lambda.) (2)
The fluorescent spectral radiance factor .beta..sub.f,I (.lambda.) is the ratio of the intensity at wavelength .lambda. of the fluorescent light component reflected by a fluorescent sample illuminated by ultraviolet light (hereinafter referred to as "UV light") and the intensity at wavelength .lambda. of the flux reflected by the complete diffuse reflection surface under the same illumination conditions, and is expressed by Equation 3. EQU .beta..sub.f,I (.lambda.)=.intg..sub.uv i.sub.I (.lambda.')e(.lambda.,.lambda.')d.lambda.'/S.sub.0 (.lambda.) (3)
The symbol i.sub.I (.lambda.) refers to the spectral intensity of the illumination light, the symbol e(.lambda.,.lambda.') refers to the efficiency of radiant fluorescent light at wavelength .lambda. excited by illumination at wavelength .lambda.' of a fluorescent sample, and symbol S.sub.o (.lambda.) refers to the intensity at wavelength .lambda. of the reflected light from a complete diffuse reflecting surface via illumination light under the same conditions.
The fluorescent spectral radiance factor .beta..sub.f,I (.lambda.) differs from the reflecting spectral radiance factor .beta..sub.o,I (.lambda.), and depends on the spectral distribution of the illumination light due to the correlation of the intensity of the UV component within the illumination light to the radiance factor at wavelength .lambda..
In the following discussion, the appended letter I used in the symbols expresses the type of white light, the appended letter D expresses the standardized D65 illuminant of the International Commission on Illumination (CIE) measuring system, and the appended letter X refers to illumination by a light source.
Although standard light D65 is typically used in evaluating fluorescent samples, the same fluorescent sample may produce a difference between the .beta..sub.f,D (.lambda.) and .beta..sub.f,x (.lambda.) in Eq. 3 if there is a difference between the spectral intensity i.sub.D (.lambda.) of the D65 illuminant and the spectral intensity i.sub.x (.lambda.) of the normal illumination light. As a result, a difference arises between .beta..sub.t,D (.lambda.) and .beta..sub.t,x (.lambda.) of Eq. 2.
Heretofore, a method is used for reconciling the spectral intensity i.sub.x (.lambda.)to the spectral intensity i.sub.D (.lambda.) by adjusting the intensity of the UV component within the illumination light ("Assessment of Whiteness and Tint of Fluorescent Substrates with Good Interinstrument Correlation," Rolf Griesser; "The Calibration of Instruments for the Measurement of Paper Whiteness," J. Anthony Bristow, Color Research and Application, Vol.19 No.6, December, 1994).
FIG. 4 is an illustration showing the construction of this conventional spectrophotometer; UV intensity is adjusted by inserting a UV cut filter capable of adjusting the degree of insertion in the flux of the illumination light to eliminate the UV component from part of the illumination flux. Equation 3 is expressed by Equations 4 in this situation. EQU .beta..sub.f,x (.lambda.)=.intg..sub.uv a.multidot.i.sub.x (.lambda.')e(.lambda.,.lambda.')d.lambda.'/S.sub.0 (.lambda.) (4)
In Eq. 4, the letter "a" refers to the attenuation of the UV component.
In FIG. 4, fluorescent sample 1 is arranged at a sample aperture 21 of integrating sphere 2. Light source 101 driven by an emitting circuit 104 includes sufficient UV component and comprises a xenon lamp; light flux 102 passes through aperture 23 and enters integrating sphere 2. A UV cut filter 103 is inserted so as to partially block the optical path of flux 102, and the flux which passes through the UV cut filter 103 has the UV component eliminated. The degree of insertion of the UV cut filter 103 is adjustable so as to allow adjustment of the UV intensity in the illumination light.
Flux 102 entering integrating sphere 2 undergoes diffuse reflection within the sphere and forms diffuse light which illuminates the fluorescent sample 1, and the radiant light 11 reflected in a predetermined direction from the illuminated surface passes through observation aperture 24 and enters sample spectral unit 5 which detects the spectral intensity. Similarly, light flux 62 having the same intensity as the illumination light of fluorescent sample 1 enters the monitoring optical fiber 61 so as to be directed to monitoring spectral unit 6 which detects the spectral intensity.
A nonfluorescent standard white light panel 12 having known spectral reflectance is arranged at aperture 21 of integrating sphere 2 to detect the spectral intensity of the illuminant of nonfluorescent standard white panel 12 and the spectral intensity of the radiant light 11 reflected from said nonfluorescent standard white panel 12.
Calculation and control unit 70 calculates the total spectral radiant factor .beta..sub.t,X (.lambda.) from the spectral intensity data input from spectroscopes 5 and 6 and based on Equation 5. ##EQU1##
In the equation, W(.lambda.) refers to the well-known spectral reflectance of the nonfluorescent standard white panel 12, S.sub.W (.lambda.) refers to the spectral intensity of the radiant light reflected from the nonfluorescent standard white panel 12, R.sub.W (.lambda.) refers to the spectral intensity of the illuminant of nonfluorescent standard white panel 12, S(.lambda.) refers to the spectral intensity of the radiant light reflected from the fluorescent sample 1, and R(.lambda.) refers to the spectral intensity of the illuminant of fluorescent sample 1.
A standard sample containing fluorescent material (hereinafter referred to as "standardized fluorescent sample") 13 is used to determine the degree of insertion of UV cut filter 103, i.e., to correct the UV intensity by determining the attenuation "a" of the UV component in Eq. 4. Standardized fluorescent sample 13 comprises paper, plastic, cloth or the like having a predetermined index of CIE whiteness under standard D65 illuminant, e.g., exhibits a degree of CIE whiteness under standard D65 illuminant when a paper is used as a standardized fluorescent sample.
The standardized fluorescent sample 13 is measured using a spectrophotometer shown in FIG. 4, and the UV intensity is corrected by adjusting the degree of insertion of UV cut filter 103 so as to match the value of CIE whiteness calculated from the obtained total spectral radiant factor .beta..sub.t,X (.lambda.) to the predetermined index of CIE whiteness.
If the adjusted UV cut filter 103 is inserted and the fluorescent sample 1 is measured when the fluorescent material contained in said sample is identical to or similar to the fluorescent material contained in the standardized fluorescent sample used to correct UV intensity, i.e., when the e(.lambda.,.lambda.') in Eq. 3 are identical or similar, the measured CIE whiteness closely approaches the CIE whiteness when fluorescent sample 1 is illuminated by standard D65 illuminant.
A second conventional example is a method for determining the total spectral radiance factor of a fluorescent sample, and is the total spectral radiance factor synthesis method described in JIS Z 8717. This method uses n individual light sources i.sub.k (.lambda.) (where k=1, 2, . . . , n) having different spectral intensities. First, a.sub.k is determined so that the value of Eq. 6 approaches the spectral intensity i.sub.D (.lambda.) of standard D65 illuminant. Next, the total spectral radiant factor .beta..sub.t,X (.lambda.) is synthesized from the total spectral radiant factor .beta..sub.t,K (.lambda.) of the fluorescent sample illuminated by each light source via Eq. 7. ##EQU2##
This method synthesizes each light source such that the spectral intensity approaches standard D65 illuminant, and can produce a total spectral radiance factor equal to standard D65 illuminant without requiring a standard fluorescent sample as in conventional art irrespective of the kind of composite material of the fluorescent sample.
In the conventional art shown in FIG. 4, time is required for measurement because measurement operation and UV cut filter movement must be repeated for adjustment of the degree of insertion of UV cut filter 103. As previously described, since UV intensity is corrected relative to a predetermined index of a standardized fluorescent sample such as CIE whiteness, the total spectral radiance factor .beta.t,X(.lambda.) may match the total spectral radiance factor .beta.t,D(.lambda.) under standard D65 illuminant at the predetermined index. However, those factors may not necessarily match each other at another index. Accordingly, other indexes calculated from the total spectral radiance factor may result in, for example, nonmatching color values.
The second example of conventional art requires the provision of many light sources having different spectral intensities to closely approach the standard D65 illuminant, and is impractical due to the complex construction and high cost.